1. Field of the Invention
The present invention relates to a current amplifier circuit whose circuit has been integrated and which is particularly used for a preamplifier circuit and the like for amplifying a current signal input from a current signal source having a high output resistance such as a photo diode.
2. Description of the Related Art
As a preamplifier circuit for amplifying a current signal input from a photo diode, various amplifier circuits described later have been heretofore used.
That is, an amplifier circuit shown in FIG. 1 receives a current signal input from a photo diode PD at an operational amplifier OA thereof, and converts the input to a voltage signal by a resistor R connected between the input and output of the operational amplifier OA and amplifies the input to output an amplified voltage signal.
Also, an amplifier circuit shown in FIG. 2 receives a current signal input from a photo diode PD at an operational amplifier OA thereof, and converts the input to a voltage signal by a diode D connected between the input and output of the operational amplifier OA and amplifies the input to output an amplified voltage signal.
Also, an amplifier circuit shown in FIG. 3A receives a current signal input from a photo diode PD at a current-mirror type current amplifier circuit 50 to amplify the current and output an amplified current signal.
Also, an amplifier circuit shown in FIG. 4A receives a current signal input from a photo diode PD as it is at an operational amplifier 60 to amplify the current and output an amplified current signal.
Usually, a photo diode has a high output resistance, and the dynamic range of a current signal input from the photo diode is as wide as 1 to 100 .mu.A.
Accordingly, for the current/voltage conversion type amplifier circuits shown in FIGS. 1 and 2, it has been difficult to accommodate for the current signal from such a photo diode to obtain a high conversion accuracy.
An amplifier circuit shown in FIG. 3A, though allows the input dynamic range thereof to be widened, has a high input impedance, so that the circuit is strongly affected by a parasitic capacity such as the junction capacity of a photo diode PD. Accordingly, as shown in an equivalent circuit of FIG. 3B, a phase delay occurs due to the time constant defined by an input resistor Rin and the parasitic capacity at the feedback input terminal of an operational amplifier, thereby causing the frequency band thereof to be limited and the operation of the operational amplifier to become unstable. That is, as shown in a frequency characteristic of FIG. 5, a gain becomes large at a high frequency region, thereby causing a high-accuracy input/output characteristic not to be obtained.
On the other hand, an amplifier circuit shown in FIG. 4A receives a current signal input from a photo diode PD at the feedback input terminal of an operational amplifier 60, so that the input impedance thereof is low, thereby causing the circuit not to be much affected by a parasitic capacity such as the junction capacity of the photo diode PD and the frequency characteristic thereof to be improved.
However, in the amplifier circuit shown in FIG. 4A, no current other than the signal current flows through a signal current detecting transistor of the operational amplifier 60, so that the frequency characteristic varies with the magnitude of the signal current, thereby causing a high-accuracy input-output characteristic not to be obtained. Also, no current other than the signal current flows through an output stage of the operational amplifier 60, so that the output impedance becomes high. Accordingly, as shown in an equivalent circuit of FIG. 4B, a phase delay occurs due to the time constant defined by an output resistor Rout and the parasitic capacity at the feedback input terminal, thereby causing the frequency band thereof to be limited and the operation of the operational amplifier 60 to become unstable.
As described above, such prior art current amplifier circuit has a problem that, where the circuit receives a current signal input from a current signal source having a high output resistance such as a photo diode to amplify the current, the magnitude of the signal current causes the frequency characteristic to be changed, or the time constant depending on the output resistance causes the frequency band to be limited and the operation to become unstable.